Sure.
Lets say 10L of rum, 35%, bought at US$150.
1. Volume of beverage * ABV (as percentage) = total volume of pure alcohol
So 10 * 0.35 = 3.5L
2. Add the total purchase price excluding tax. Convert to AUD.
AUD$212
3. Look up the customs tarrif for beverage of choice. Multiply that by the volume.
Link is here: Chapter 22
For rum, it's $85.36/L (again, this is L of pure alcohol, not L of rum)
So, 85.36 * 3.5 = $298.76.
Some tarrifs include a value percentage as well (ie, $80/L + 5%). If that's the case, add that component too.
For rum, this would be an additional $10.60. However, there's lots of national exceptions, which will exclude this. This component is not applicable to US rum.
4. Add the numbers for part 3, that is the total duty.
So $298.76
5. Add this to the value of the goods.
$298.76 + 212
= $510.76.
6. Calculate GST at 10% of part 5. That is the total GST.
=$51.07
7. Add parts 4 and 6 together. That's the total payable.
=$349.83.
Through the mail they can also charge you an assessment fee (they also count the shipping cost in the calculation of GST). I don't believe they do that in person and they didn't charge me.
As for number 2 - I buy suitcases and pillows from Walmart. Two enemies - movement and glass to glass contact. Wrap each bottle in clothes, pack suitcase tight with protection on all sides (a pillow underneath and above then stuff around the sides). Don't put more than 5L of alcohol in each suitcase (if it's about 23% that's actually illegal as it's dangerous goods).
For the larger loads I will have 4-5 suitcases. For these I obviously need a friend, both for the baggage allowance (though being QF WP helps) but also for the 5L rule. Technically that's per person.